1. Field of the Invention
The present invention relates to a self-contained cooler/freezer apparatus, and more particularly, to a self-contained cooler/freezer apparatus which utilizes solid carbon dioxide in the form of blocks or snow to maintain a predetermined temperature within the apparatus.
2. Discussion of the Prior Art
Many shipping and trucking lines use refrigerated containers to carry perishable commodities over long distances. Typically, such containers are designed to carry either frozen foods or foods that must be maintained at higher, but still refrigerated temperatures, such as for example 40 degrees Fahrenheit. There exists a multitude of portable refrigeration devices designed to maintain or preserve perishable commodities at a given temperature for a given period of time. These refrigeration devices utilize various means to maintain the commodities at a given temperature, including compressed gas refrigeration systems, liquid cooled refrigeration systems, and solid cooled refrigeration systems.
An example of a refrigeration system employing compressed gas is set forth in U.S. Pat. No. 3,633,381. U.S. Pat. No. 3,633,381 discloses a portable refrigerator employing an open cycle system. A stored compressed gas, such as carbon dioxide, is passed from a storage container through an evaporator. The evaporator comprises a serpentine passageway for the gas in a surrounding medium such as water, which is maintained frozen due to the passage of the expanding compressed gas through the coiled passageway. The temperature of the evaporated medium is lower than the ambient temperature of the interior of the container comprising the storage portion of the refrigerator which is cooled thereby. The gas passing through the evaporator may be exhausted into the interior of the container whereby the cooler air which is next to the evaporator medium is circulated throughout the interior of the container.
U.S. Pat. No. 3,961,925 discloses a portable self-contained refrigerated storage and transportation container for preserving perishable commodities, and includes an insulated storage chamber for the perishable commodities. A recirculating liquid cooling system is provided within the container and includes conduit and nozzle means disposed within the storage chamber and adapted to spray a liquid coolant, such as chilled brine, directly onto the perishable commodities to maintain them at a uniform cooled temperature. The sprayed liquid coolant is collected in the bottom portion of the storage chamber. A closed refrigeration system is also provided within the container and includes heat exchange means disposed within the bottom portion of the storage chamber for cooling the sprayed liquid coolant which has collected there.
In U.S. Pat. No. 4,502,293, there is disclosed a solid carbon dioxide cooling container. The container includes an insulated top, bottom, opposite sides and opposite end walls. An upstanding transverse insulated hollow housing is mounted within the container adjacent one end thereof and a carbon dioxide snow cabinet constructed from a "good" heat transfer material is disposed within the housing with opposing wall portions of the cabinet and housing passing exteriorly about the cabinet. A heat insulative horizontal baffle is mounted within the container spaced below the top wall and extends between the sidewalls thereof. The baffle defines a cooled air passage beneath the top wall extending lengthwise of the container. The airflow passage includes an outlet end adjacent and in at least reasonably closed communication with the end of the cooled air passage adjacent the aforementioned one container end wall and an inlet end opening outwardly of the housing into the interior of the container below the baffle. The end of the cooled air passage adjacent the other container end wall opens into the interior of the container and a thermostatically controllable air pump structure is provided to effect airflow inwardly of the inlet of the airflow passage, through the airflow passage and into the cooled air passage. In addition, a structure is provided for spray discharging of liquid carbon dioxide into the interior of the upper portion of the cabinet and into the airflow passage at points spaced in order to form carbon dioxide snow thereon.
The above described patents are representative of the various systems available for preserving perishable items. Each of these systems offers varying degrees of cooling capacity, temperature control and temperature control system simplicity. However, none of the above described systems alone offers a self-contained cooler/freezer apparatus which provides a high cooling capacity and a highly accurate and simplistic temperature control system.
U.S. Pat. No. 4,276,752 discloses a refrigerated cargo container which utilizes solid carbon dioxide as a cooling medium. The refrigerated cargo container comprises a bunker which is filled with solid carbon dioxide or dry ice, a heat exchanger which is in thermal contact with the solid carbon dioxide, a fan, and ducts for circulating carbon dioxide gas through the container. Warm gas from the container's interior and the cargo contained therein rises to the top of the container due to the natural convective flow of gas in the container. This warm gas enters the heat exchanger and causes the solid carbon dioxide to sublime. As the coolant sublimes, the heat exchanger is cooled, and as warm gas passes over this cooled heat exchanger that gas is likewise cooled. A fan can be installed to increase the flow of warm gas from the interior of the container to the heat exchanger. A damper means is located in the duct carrying cold gas from the heat exchanger to control the amount of cool gas entering the container. A control means may also be installed to control the operation of the fans based on temperature differentials.
The above described patent utilizes natural convection of gas within the container in conjunction with a heat exchanger to provide a flow of cooling gas. A fan and damper means are utilized to augment air flow and partially control the circulation of the cooling gas. However, the use of a heat exchanger in direct contact with the dry ice causes pockets of carbon dioxide gas to form as the dry ice sublimates. These pockets create a large thermal resistance between the warmed gas and the dry ice heat sink, thereby limiting the heat rejection capability of the system. Limiting the heat rejection capability prevents the maintenance of lower temperatures within the cargo container. Additionally, in relying solely on natural convection, there is a diminished ability to accurately control and maintain the temperature within the container.